1. Field of the Invention
The present invention relates to a heat treatment apparatus which emits a flash of light to expose a substrate including a semiconductor wafer, a glass substrate for a liquid crystal display device and the like to the flash of light, thereby heating the substrate.
2. Description of the Background Art
Conventionally, a lamp annealer employing a halogen lamp has been typically used in the step of activating ions in a semiconductor wafer after ion implantation. Such a lamp annealer carries out the activation of ions in the semiconductor wafer by heating (or annealing) the semiconductor wafer to a temperature of, for example, about 1000° C. to about 1100° C. Such a heat treatment apparatus utilizes the energy of light emitted from the halogen lamp to raise the temperature of a substrate at a rate of about hundreds of degrees per second.
In recent years, with the increasing degree of integration of semiconductor devices, it has been desired to provide a shallower junction as the gate length decreases. It has turned out, however, that even the execution of the process of activating ions in a semiconductor wafer by the use of the above-mentioned lamp annealer which raises the temperature of the semiconductor wafer at a rate of about hundreds of degrees per second produces a phenomenon in which the ions of boron, phosphorus and the like implanted in the semiconductor wafer are diffused deeply by heat. The occurrence of such a phenomenon causes the depth of the junction to exceed a required level, giving rise to an apprehension about a hindrance to good device formation.
To solve the problem, there has been proposed a technique for exposing the surface of a semiconductor wafer to a flash of light by using a xenon flash lamp to raise the temperature of only the surface of the semiconductor wafer implanted with ions in an extremely short time (several milliseconds or less). This technique is disclosed, for example, in U.S. Pat. Nos. 6,998,580 and 6,936,797. The xenon flash lamp has a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamp is shorter than that of light emitted from the conventional halogen lamp, and approximately coincides with a basic absorption band of a silicon semiconductor wafer. It is therefore possible to rapidly raise the temperature of the semiconductor wafer, with a small amount of light transmitted through the semiconductor wafer, when the semiconductor wafer is exposed to a flash of light emitted from the xenon flash lamp. Also, it has turned out that a flash of light emitted in an extremely short time of several milliseconds or less can achieve a selective temperature rise only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short time by using the xenon flash lamp allows the execution of only the ion activation without deeply diffusing the ions.
In the heat treatment apparatus which performs flash heating as described above, air is supplied into and exhausted from a lamp house including a xenon flash lamp provided therein for the purpose of cooling down the xenon flash lamp. Thus, the heat within the lamp house is exhausted, and the xenon flash lamp itself is cooled down by the air.
Many heat treatment apparatuses which perform flash heating are provided with a hot plate for preheating a semiconductor wafer before exposing the semiconductor wafer to a flash of light. An attempt to heat the surface of the semiconductor wafer up to 1000° C. or higher only by exposing the semiconductor wafer to the flash of light requires the emission of a flash of light having an extremely large amount of energy. This causes the rapid heating of only the wafer surface to give rise to an apprehension about a crack in the semiconductor wafer. To prevent this, the hot plate is used to preheat the semiconductor wafer placed thereon up to about 500° C., and thereafter a flash of light is emitted from the xenon flash lamp to cause the temperature of the semiconductor wafer to reach a predetermined annealing temperature.
However, infrared radiation of a relatively long wavelength depending on the temperature of the hot plate emanates from the hot plate. The discharge tube (made of quartz) of the xenon flash lamp is externally heated by the radiant heat from the hot plate. If the xenon flash lamp is heated by the radiant heat from the hot plate at a relatively high temperature of 500° C. or higher, the xenon flash lamp is not sufficiently cooled down only by the above-mentioned supply and exhaust of air, but there arise problems such that the color change of the discharge tube reduces the lifetime of the xenon flash lamp and such that the deterioration of the xenon flash lamp proceeds rapidly to result in the rupture of the xenon flash lamp in the worst case. In the conventional techniques, the upper limit of the preheating temperature attained by the hot plate is hence restricted, and the treating conditions of the annealing process of the semiconductor wafer are also inevitably limited.
A lamp house having a xenon flash lamp provided therein includes a quartz plate which allows a flash of light emitted from the xenon flash lamp to pass therethrough and which serves as a lamp light radiation window. The lamp light radiation window is also heated by far infrared radiation of a long wavelength emanating from the hot plate. If air is supplied into and exhausted from the lamp house for the purpose of cooling down the xenon flash lamp, the lamp light radiation window is also cooled down, and the distribution of temperature thereof becomes nonuniform. The infrared radiation of a long wavelength emanates also from the heated lamp light radiation window to influence the temperature of the semiconductor wafer W. The nonuniformity of the temperature distribution across the lamp light radiation window might cause variations in the temperature distribution across a semiconductor wafer to be treated.